1. Field of the Invention
This invention relates to a power source device, and more particularly to a power source device used for such as a watthour meter.
2. Description of the Related Art
FIG. 7 shows a prior art example of such a power source device.
In this Figure, this power source device is equipped with: a terminal VP1 that inputs a primary DC positive voltage vp1 of positive polarity obtained by rectifying and smoothing AC voltage, a terminal VM1 that inputs a primary DC negative voltage vm1 of negative polarity obtained by rectifying and smoothing AC voltage, a terminal GND that inputs a ground potential gnd, a reference voltage generating device 11 that generates a reference voltage vref, a positive voltage generating device 12 that generates a positive-polarity stabilized DC positive voltage vp2, and a negative voltage generating device 13 that generates a negative-polarity stabilized DC negative voltage vm2.
Reference voltage generating device 11 generates reference voltage vref, taking as the positive power source voltage primary DC positive voltage vp1 and as negative power source voltage ground potential gnd.
Positive voltage generating device 12 is equipped with an operational amplifier OP1, a resistor R11, and a resistor R12, to constitute a noninverting amplifier. Positive voltage generating device 12 applies noninverting amplification to reference voltage vref supplied from reference voltage generating device 11, to generate DC positive voltage vp2 as follows. EQU vp2=(1+R2/R1).times.vref
This DC positive voltage vp2 is output from a terminal VP2.
Negative voltage generating device 13 is equipped with an operational amplifier OP12, a resistor R16, and a resistor R17, to constitute an inverting amplifier. Negative voltage generating device 13 applies inverting amplification to DC positive voltage vp2 supplied from positive voltage generating device 12, to generate DC negative voltage vm2 as follows. EQU vm2=(-1).times.R7/R6.times.vp2
This DC negative voltage vm2 is output from a terminal VM2. The above-described power source device is connected to a LSI 5 which constitutes a part of a watthour meter, such that as the positive and negative power sources DO positive voltage vp2 and DC negative voltage vm2 are applied, respectively.
However, with the conventional power source device, in the case where reference voltage generating device 11 and LSI 5 (that employs as the positive power source DC positive voltage vp2, and as negative power source DC negative voltage vn2) are integrated with a view to cost reduction, the power source of reference voltage generating device 11 has to be isolated from the power source of LSI 5. In order to isolate the power sources of reference voltage generating device 11 and LSI 5, it is necessary to make an LSI 10B a twin tab process as shown in FIG. 8, or to make it by a triple well process as shown in FIG. 9 or FIG. 10, and this in fact ends up increasing costs.